This invention relates to azametallocene catalysts useful for polymerizing olefins such as ethylene and other unsaturated monomers. In particular, it relates to catalysts having at least one ligand that contains a pyrrolyl ring bonded to a transition metal.
Until recently, polyolefins have been primarily made with conventional Ziegler catalyst systems. These catalysts typically consist of transition metal-containing compounds and one or more organometallic compound. For example, polyethylene has been made using Ziegler catalysts such as titanium trichloride and diethylaluminum chloride, or a mixture of titanium tetrachloride, vanadium oxytrichloride, and triethylaluminum. These catalysts are inexpensive but they have low activity and therefore must be used at high concentrations. As a result, it is sometimes necessary to remove catalyst residues from the polymer, which adds to production costs. Neutralizing agents and stabilizers must be added to the polymer to overcome the deleterious effects of the catalyst residues. Failure to remove catalyst residues leads to polymers having a yellow or grey color and poor ultraviolet and long term stability. For example, chloride-containing residues can cause corrosion in polymer processing equipment.
Furthermore, Ziegler catalysts produce polymers having a broad molecular weight distribution, which is undesirable for some applications such as injection molding. They are also poor at incorporating .alpha.-olefin comonomers. Poor comonomer incorporation makes it difficult to control the polymer density. Large quantities of excess comonomer may be required to achieve a certain density and many higher .alpha.-olefins, such as 1-octene, may be incorporated at only very low levels, if at all.
Although substantial improvements in Ziegler catalyst systems have occurred since their discovery, these catalysts are now being replaced with the recently discovered metallocene catalyst systems. A metallocene catalyst typically consists of a transition metal compound which has one or more cyclopentadienyl ring ligands. They have low activities when used with organometallic compounds, such as aluminum alkyls, which are used with traditional Ziegler catalysts, but very high activities when used with aluminoxanes as cocatalysts. The activities are generally so high that catalyst residues need not be removed from the polymer. Furthermore, they produce polymers with high molecular weights and narrow molecular weight distributions. They also incorporate .alpha.-olefin comonomers well. However, at higher temperatures metallocene catalysts tend to produce lower molecular weight polymers. Thus, they are useful for gas phase and slurry polymerizations of ethylene, which are conducted at about 80.degree. C. to about 95.degree. C., but they do not generally work well in solution polymerizations of ethylene, at about 150.degree. C. to about 250.degree. C. The polymerization of ethylene in solution is desirable because it allows great flexibility for producing polymers over a wide range of molecular weights and densities as well as the use of a large variety of different comonomers. One can produce polymers that are useful in many different applications, for example, high molecular weight, high density polyethylene film useful as a barrier film for food packaging and low density ethylene copolymers with good toughness and high impact strength.
One of the characteristics of metallocene catalysts is the presence of .pi.-bonds between one or more cyclopentadienyl ring-containing ligands and a transition metal. These bonds are moderately strong and stable. In contrast, .pi.-bonds between transition metals and pyrrolyl ring-containing ligands are relatively unstable. Although these nitrogen-containing ligands form unstable .pi.-bonds, analogous ligands with other Group VA elements (P, As) form stable bonds. King (Inorg. Chem., 1964, 3, 796) and Joshi (J. Organomet. Chem., 1964, 1, 471) found that the pyrrolyl analog of ferrocene was less stable and had indications that pyrrolyl ligands tended to form .sigma.-bonds. Van Bynum et al. prepared 2,5-dimethylpyrrole derivatives of Zr and showed that there was no .pi.-bonding to the Zr atoms in these compounds (Can. J. Chem., 1986, 64, 1304). Ladipo et al. showed that indole complexes with iridium also formed .sigma.-bonds to the transition metal (Inorg. Chem., 1990, 29, 4172). A recent review of the subject of .pi.-bonded pyrrolyl-containing ligands (J. Zakrezewski, Heterocycles, 1990, 31, 383) reemphasized the instability of these types of complexes.